1. Field of the Invention
This invention relates to processes for rendering particles of tacky ethylene-vinyl acetate (hereinafter referred to as "EVA") copolymers agglomeration-resistant. The invention also relates to processes for deagglomerating agglomerated particles of EVA copolymers and rendering same agglomeration-resistant. It also relates to processes for forming particles of heretofore non-pelletizable ethylene-vinyl acetate copolymers in liquid media capable of rendering the particles agglomeration-resistant, and thereby, the copolymer pelletizable.
In accordance with the invention these results are obtained by the formation of at least a partially hydrolyzed surface on the ethylene-vinyl acetate particles treated.
2. Description of the Prior Art
It is well known in the art that various polymers, when in particulate form, tend to block and stick together during storage. For example, in U.S. Pat. No. 3,753,965, it is disclosed that ethylene-acrylic acid copolymers in the particulate form, especially those copolymers containing at least 10% by weight of acrylic acid, tend to block and stick together when warm and under the pressure generated by the weight of the copolymer in storage. This reference teaches that it is possible to render the particles less prone to blocking and sticking by treating them with a base until as little as 2.5% by weight of the carboxyl groups on the particle surfaces are converted to the corresponding salt groups.
It is also known that high molecular weight EVA copolymers which contain up to about 35% by weight of vinyl acetate (hereinafter referred to as "VA") readily lend themselves to pelletization by the methods used industrially for polyethylene resin, such as strand-chopping and pelletization in a fly-knife cutter bath. The term, "strand-chopping" as used herein means pelletization performed, for example, in a Cumberland size 6 pelletizer (Cumberland Engineering Co.). The term pelletization in a "fly-knife cutter bath" as used herein means pelletization of a resin as carried out, for example, in a Black-Clawson size No. 1 Pelletor.
However, those high molecular weight EVA polymers which contain substantially more than about 35% by weight of VA present serious problems in this regard. Copolymers containing about 35 to 60% by weight of VA, referred to herein as tacky, pelletizable copolymers, form pellets that not only stick together when warm or under the pressure generated by the weight of the resin in storage, as is observed in the case of the ethylene-acrylic acid copolymer, but also, at the higher Va contents within the range, readily stick together even at room temperature, soon after they are pelletized, to produce agglomerated bodies that eventually may aggregate further until they form a single, relatively coherent mass which takes on the shape of the container in which they are stored. However, pellet-boundaries are usually distinguishable in such agglomerates.
EVA copolymers which contain more than about 60 weight percent VA can not be pelletized by conventionl means because the pellets thereof immediately coalesce upon formation to produce a substantially homogeneous mass of resin in which pellet-boundaries can scarcely be distinguished; such EVA copolymers are referred to herein as tacky, non-pelletizable EVA copolymers.
It will be recognized by those skilled in the art that the aforesaid composition ranges may be somewhat arbitrary, and will depend to some degree upon such factors as the molecular weight and structure of the copolymers as well as on the content of residual volatile materials (VA monomer, etc.) and on the conditions of storage of already pelletized copolymers, such as ambient temperature and pressure due to their weight.
It is an object of this invention to overcome these and other problems by providing processes for rendering such EVA copolymers agglomeration-resistant. In particular it is an object of this invention to provide processes for rendering tacky, pelletizable EVA copolymers agglomeration-resistant before, during, or after the mechanical operations whereby the copolymers are converted into the pellet or particulate form. Additionally, when the tacky, pelletizable copolymers in the particulate or pelletized form have already agglomerated, it is a further object of this invention to provide a process in which such agglomerates are deagglomerated to release the original particles or pellets in the agglomeration-resistant form.
It is a further object of this invention to provide processes for rendering tacky, heretofore non-pelletizable EVA copolymers agglomeration-resistant before or during mechanical operations thereon, whereby such copolymers may be converted to the particulate or pellet form, to thereby provide the heretofore unpelletizable EVA copolymer in an agglomeration-resistant, and hence pelletized or particulate form.
Complete and partial alcoholysis of EVA copolymers in the particulate form is well known in the art (U.S. Pat. No. 3,510,463 and British patent specification No. 1,095,204). Although it is disclosed (British patent specification No. 1,095,204) that the reaction can be confined to the surface of the particulate or shaped form of the copolymer to impart antistatic properties or improved receptivity to printing inks, nevertheless it has not been shown in these teachings that such surface treatment of the particles with a base will render them more resistat to blocking and sticking together.
According to U.S. Pat. No. 3,753,965, the improved blocking- and sticking-resistance observed for base-treated ethylene-acrylic acid copolymer particles is caused by the protective action of carboxylic acid salt groups formed on the particle surfaces by neutralization of the surface carboxylic acid groups. However, it would not be expected that EVA copolymer particles would be rendered resistant to blocking and sticking as a result of a similar base treatment, because the methyl acetate or sodium acetate formed by such treatment would be substantially washed off the surface of the particles, either by the basic liquid media employed or by water used later to wash off any excess base.
In the EVA pellet alcoholysis processes disclosed in the above-mentioned references, it is taught that the direct alcoholysis of EVA is best effected with EVA copolymers which contain less than about 40% by weight of VA. Thus, according to U.S. Pat. No. 3,510,463, if the copolymer pellets contain more than about 40% by weight of VA, they will dissolve in the liquid alcoholysis media disclosed, e.g., mixtures of methanol and toluene or methylene chloride. Therefore, pellets of those EVA copolymers which are of principal interest in the present invention, i.e., those which contain more than about 35% by weight of VA, could not in most cases be surface-treated with a base by the process of U.S. Pat. No. 3,510,463 because they would dissolve. Moreover, even though British patent specification No. 1,095,204 broadly discloses EVA copolymers containing up to 50% by weight of VA as being operable in a pellet-alcoholysis process which is practically identical to that of U.S. Pat. No. 3,510,463, the preferred range disclosed therein is 5-25% by weight of VA.
It is further known (Japanese Pat. No. 70 12,239; Chem. Abstrs. 73; 88645e) that the alcoholysis of EVA film in methanolic sodium methoxide yields a blocking-resistant, fully hydrolyzed film; but this process is apparently very slow, since a reaction time of 1 hour at 20.degree. C is disclosed. Moreover, and more importantly, complete alcoholysis of the film is contemplated. As will appear hereinafter, this differs markedly from the present invention.
In addition, U.S. Pat. Nos. 2,386,347 and 2,339,653 disclose that EVA copolymers containing at least 40% by weight of VA are partially hydrolyzed (or alcoholyzed) by treatment with an alkali to yield a material having wet strength and low temperature toughness. In the first of these patents the degree of partial hydrolysis contemplated is a minimum of 80%, while in the second patent, the partial hydrolysis contemplated is from 10 to 80%. In either case, a substantial degree of hydrolysis of the EVA copolymer is contemplated. As will appear below, the present invention contemplates a very minor degree of hydrolysis of the EVA copolymer.
In contrast to these prior art processes, and as will be discussed in more detail below, the processes of this invention contemplate extremely rapid surface treatments of EVA copolymers to effect the hydrolysis of the VA groups on or near the surface of the particles without any significant hydryolysis of the interior. Thus, according to the processes of this invention, the conditions of EVA stock temperature, the composition and temperature of the treating medium, and the concentration of the alkali in the medium are controlled so that sufficient agglomeration-resistance is imparted in the shortest possible contact time (a few seconds to a few minutes). Thus the hydrolysis will proceed very rapidly on the surface of the EVA copolymer and then essentially stop, so as to prevent significant hydrolysis from occurring within the pellets.
As has already been pointed out, the known processes are not generally useful for the surface treatment of EVA copolymers containing more than about 35% by weight of VA since it is desirable that the EVA copolymer not dissolve, but more importantly that the surface treatment be accomplished rapidly without substantial overall hydrolysis.
Moreover, there is no teaching in any of the heretofore known art that the treatment of already agglomerated pellets with a base can reverse the agglomeration process, so that the individual pellets are substantially regenerated from the agglomerates, and are at the same time additionally made sufficiently more resistant to blocking and sticking to prevent subsequent reagglomeration either at room temperature or at the temperatures and pressures encountered in normal use. In fact, it would logically be expected by those skilled in the art that such treatment of pellet-agglomerates with a base would merely render the agglomerates themselves resistant to sticking and blocking. The regeneration of the original, individual pellets from the pellet agglomerates is quite unexpected and surprising.